Hello,
recently I couldn't resist to acquire one of these Racal MA259 units
with a SA 500 crystal oscillator which was designed by Sulzer.
What a beautiful piece of ancient high precision laboratory gear!
Well, it wasn't claimed to still meet the specifications, but it was at
least in a working condition.
First I was surprised about the high frequency drift that was about 50
times as high as specified, that is, a few 1E-8's rather than the
impatiently awaited 5E-10 per day.
I could however observe the drift slowly getting down, and, after
reading the manual, I was less concerned as it pointed out that the
specified accuracy can be expected to be met after a full month of
continuous operation. That was before I realized that the inner oven
wasn't heating at all, and its temperature being low by a few degrees.
So, I gotta have a look inside that huge cylinder with the large dewar
flask inside. Unfortunately, the toroid transformer of the temperature
bridge had developed a short beween the primary and secondary windings.
I could burn the short out, but there is now some noise visible on the
meter needle as well as, much slower of course, on the frequency
stability plot. There might still be some conductivity left between the
windings, or something else is noisy.
Next I noticed that the meter reading of the oscillator amplitude was
quite a bit off. I was already suspecting those infamous carbon
composite resistors might have drifted as usual. So, I decided to
replace every resistor in the oscillator, the buffer / amplitude control
stage and the inner oven. Indeed, several of the resistors that set the
oscillator amplitude were drifted by up to 100 percent and were even
noisy on the ohmmeter. Probably not that great for a ultra high
precision circuit... Well, ahould I have replaced the capacitors, too??
Now the amplitude is right, and the drift is starting to settle again.
For the time being, I want to see how it stabilizes. Then come the next
project to take care of the inner oven noise, and to fix the 100 kHz
divider that has stopped working.
Is there any information about the Sulzer desiagn available? Anyone who
repaired one of these oscillators?
Adrian
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